1. Field of the Invention
The present invention relates to signal light amplification utilizing a nonlinear optical effect.
2. Description of the Related Art
Communication apparatuses and communication systems utilizing optical technology have come into wide use as communication capacities and transmission distances have increased. In optical communication, transmission speed (data bit rate), total transmission capacity through a single optical fiber ((transmission rate per channel)×(number of channels)), and transmission distance are limited by waveform distortion or phase distortion of signal light, optical S/N (signal-to-noise) ratio of signal light, etc.
Waveform distortion or phase distortion of signal light is caused by chromatic dispersion (including high-order dispersion and polarization-mode dispersion), nonlinear optical effect, etc., occurring in an optical fiber forming a transmission path. To cope with waveform distortion caused by chromatic dispersion, a transmission path equipped with a normal dispersion fiber and an anomalous dispersion fiber arranged alternately, and a dispersion compensation technique using such a chromatic dispersion compensator as dispersion compensating fiber are employed.
To cope with signal light loss in an optical fiber, a technique using an optical amplifier, such as an optical fiber amplifier, is employed. Optical S/N ratio varies depending on a decrease in power due to signal light loss in an optical fiber line, amplified spontaneous emission (ASE) noises generated from the compensation of signal light loss by optical amplifier, noises generated in a receiver/transmitter, etc.
Today, a problem of grave concern is the realization of long distance transmission of signal light transmitted at a high transmission speed of 40 Gb/s, 100 Gb/s or 160 Gb/s. In high-speed transmission, however, even if a combination of a high-precision chromatic dispersion compensator and a high-quality optical amplifier is provided, reduction in the S/N ratio of signal light remains significant because of residual waveform distortion, phase distortion, and ASE noises generated from the optical amplifier. For this reason, a practical fiber transmission distance of signal light transmitted at 40 Gb/s is limited to several hundred km, and that of signal light transmitted at 160 Gb/s is limited to several km.
For the realization of long distance transmission of such high-speed signal light, it is essential to achieve an optical signal processing apparatus capable of reshaping distorted waveforms and phases, and suppressing accumulated ASE noises and phase noises. To meet this demand, optical signal processing apparatuses that controls the waveform of signal light using an optical limiter function have been disclosed such as those disclosed in, for example, Japanese Patent Application Laid-Open Publication Nos. 2000-31901 and 2000-49703.
The optical signal processing apparatus receives signal light and pump light (pulsed pump light) to a nonlinear optical medium, such as an optical fiber. The optical signal processing apparatus adjusts the relative power of signal light and pump light to saturate signal light gain resulting from a nonlinear optical effect, and thus suppresses noise in signal light having an intensity level of “1”.
The conventional optical signal processing apparatus above, however, poses a problem in that when light pulses are used as pulsed pump light, signal light cannot be amplified uniformly if the timing of signal light and the light pulses do not match. Synchronizing the signal light and the light pulses requires a clock recovery circuit, etc. In this case, different clock recovery circuits are needed according to the modulation method, bit rate, pulse width, etc., of the signal light.
Therefore, to cope with multiple types of signal light, multiple clock recovery circuits are needed, which leads to a problem of a larger and more complicated optical signal processing apparatus that invites a cost increase. To solve this problem, continuous light may be used as pump light. However, the efficiency of the occurrence of the nonlinear optical effect depends on the peak power of pump light bringing about a problem in that a large output linear optical amplifier that increases the overall power of continuous light is needed to ensure a sufficient gain by raising the efficiency of occurrence of the nonlinear optical effect.
For example, when an optical fiber is used as a nonlinear optical medium, attempts to increase the power of continuous light causes stimulated Brillion scattering in the optical fiber, thereby resulting in a part of the continuous light being reflected. Therefore, even if the power of the continuous light is increased by using a large output linear optical amplifier, sufficiently increasing the power of continuous light in the optical fiber is difficult, posing a problem in that sufficient gain cannot be ensured when continuous light is used as pump light.
When signal light is in the form of a wavelength-division multiplexed (WDM) signal, signal light in each of channels of the WDM signal arrives in random timing. For this reason, to carry out waveform reshaping of signal light for each channel, the WDM signal must be branched according to channel to reshape the waveform of each branch signal light individually. This requires multiple pump light generating circuits, clock recovery circuits, etc., each respectively corresponding to each channel, thus raising a problem of a larger and more complicated optical signal processing apparatus, inviting cost increases.